Digital radio communication devices conventionally transfer information by modulating a carrier wave using a baseband signal that corresponds to the information to be transmitted.
A variety of digital modulation methods have been used to modulate the carrier wave. With ASK (Amplitude Shift Keying), the amplitude of the carrier wave signal changes according to the baseband signal. With FSK (Frequency Shift Keying), the frequency of the carrier wave is changed. With PSK (Phase Shift Keying), the phase of the carrier wave is changed. With QAM (Quadrature Amplitude Modulation) the amplitude and phase of the carrier wave are independently changed.
Radio communication devices that receive signals that have been modulated using the digital modulation methods described above demodulate the received signals by performing the opposite of the modulation processing to obtain the original data.
When digital modulation methods are used for mobile communication, it is well known that reception characteristics can be seriously affected by fading, a name given to wild fluctuations in the reception level due to the reflection and scattering of radio waves.
Diversity reception methods are effective in compensating for the decreases in reception level due to fading. Diversity reception methods have the same signal received via a plurality of reception systems and combine a plurality of received signals to produce a high-quality reception signal.
Several diversity reception methods are available. A selection combining method selects the received signal with the highest level out of the signals received by each reception system and demodulates the selected reception signal. An equal-gain combining method combines the reception signals of each reception system at the same level and demodulates the resulting signal. A maximal-ratio combining method gives a weighting to the reception signal of each reception system in proportion to its relative reception level, combines the weighted reception signals, and demodulates the result.
Of the above methods, the maximal-ratio combining method combines signals after assigning higher weightings to signals with higher reception levels. This means that signals with lower levels of noise are assigned higher weightings, so that reception signals are effectively combined by this method.
The following is a description of a diversity receiving apparatus that uses the maximal-ratio combining method.
FIG. 8 shows the construction of a diversity receiving apparatus that uses the maximal-ratio combining method.
This diversity receiving apparatus is a reception device that receives transmission data which has been modulated using QPSK. The diversity receiving apparatus decodes the transmission data by performing QPSK demodulation and by combining signals that are weighted according to their relative reception levels.
QPSK refers to a transmission method for two-bit (four-value) information. First, two orthogonal carrier waves are separately subjected to phase modulation according to a two-bit baseband signal. The two modulated signals resulting from the modulation are then added and transmitted. A reception device performs the opposite processing to the QPSK modulation and so obtains two-bit (four-value) information for each symbol.
As shown in FIG. 8, the diversity reception apparatus includes phase demodulating units 329˜332, I component ROMs 317˜320, Q component ROMs 321˜324, an I component adder 325, a Q component adder 326, a judging unit 327, and a clock generating unit 328.
The phase demodulating units 329˜332 detect phase differences between the phase of the received reception signal and the phase of the immediately preceding symbol. The phase demodulating units 329˜332 output the results as phase data θk (where k=1, 2, 3, 4). Here, the values k=1, 2, 3, 4 respectively correspond to the phase demodulating units 329, 330, 331, and 332.
In detail, the phase demodulating unit 329 includes the input terminal 301, the phase detecting unit 305, the phase delaying unit 309, and the phase adding unit 313. This composes a phase detection-type delay detection device corresponding to the PSK (Phase Shift Keying) modulation method.
The input terminal 301 is a terminal through which a reception signal, which has been digitized by an A/D converter or a limiter, is inputted into the present diversity receiving apparatus.
The phase detecting unit 305 compares the phase of the reception signal inputted via the input terminal 301 with the phase of a local oscillator (not illustrated) and outputs the detected phase value that has been digitized. This means that the phase detecting unit 305 detects only the phase component of the reception signal. Since the amplitude component of the reception signal is not required by the phase detecting unit 305, a linear amplifier is not required.
The phase delaying unit 309 delays the detected phase outputted by the phase detecting unit 305 by the time equivalent to one symbol, and outputs the result as the delayed phase.
The phase adding unit 313 detects the phase difference between the detected phase and the delayed phase and outputs this as the phase data θ1.
The phase demodulating units 330˜332 have the same construction as the phase demodulating unit 329 and so output the phase data θ2˜θ4.
The I component ROMs 317˜320 and the Q component ROMs 321˜324 respectively correspond to the phase demodulating units 329˜332. Using the combining coefficients Rk (k=1, 2, 3, 4) and the phase data θk (k=1, 2, 3, 4), these ROMs output the in-phase components Rk2*cos θk (k=1, 2, 3, 4) of the reception signals and the quadrature components Rk2*sin θk (k=1, 2, 3, 4) of the reception signals that have both been weighted using the combining coefficients Rk.
These combining coefficients Rk are signals showing the signal levels (RSSI: Received Signal Strength Indicator) that have been detected by a high-frequency receiving unit (not illustrated) in each phase demodulating unit 329˜332.
The I component ROMs 317˜320 store beforehand the input reception signals Rk2*cos θk (k=1, 2, 3, 4) for the in-phase components for every possible combination of the combining coefficients Rk (k=1, 2, 3, 4) and phase data θk (k=1, 2, 3, 4).
Both the combining coefficients Rk and phase data θk are 8 bits long. The I component ROMs 317˜320 store calculation results for all combinations of the 28 different values of Rk and the 28 different values of θk, which is to say 216 different calculation results. When the combining coefficients Rk and different values of the phase data θk are inputted, the I component ROMs 317˜320 output the values of Rk2*cos θk corresponding to the inputted combination.
In the same way, the Q component ROMs 321˜324 store beforehand the input reception signals Rk2*sin θk (k=1, 2, 3, 4) for the quadrature components for every possible combination of the combining coefficients Rk (k=1, 2, 3, 4) and phase data θk (k=1, 2, 3, 4). When the combining coefficients Rk and different values of the phase data θk are inputted, the Q component ROMs 321˜324 output the values of Rk2*sin θk corresponding to the inputted combination.
The I component adder 325 combines the weighted in-phase components Rk2*cos θk (k=1, 2, 3, 4) of the reception signals that have been outputted by the I component ROMs 317˜320 and outputs the combined in-phase components of the reception signals.
The Q component adder 326 combines the weighted quadrature components Rk2*sin θk (k=1, 2, 3, 4) of the reception signals that have been outputted by the Q component ROMs 321˜324 and outputs the combined quadrature components of the reception signals.
The clock generating unit 328 extracts symbol sections from the combined in-phase components and quadrature components of the reception signals outputted by the I component adder 325 and the Q component adder 326. Based on the extracted symbol sections, the clock generating unit 328 generates a clock that serves as the standard for the judgement timing of the judging unit 327.
The judging unit 327 outputs two-bit (four value) data by judging whether the combined in-phase components and quadrature components of the reception signals outputted by the I component adder 325 and the Q component adder 326 are positive or negative. The judging unit 327 performs these judgements in synchronization with the clock generated by the clock generating unit 328.
In this way, a diversity receiving apparatus that uses the conventional maximal-ratio combining method decodes data from a reception signal.
There are cases with conventional diversity receiving apparatuses, however, where the extraction timing of symbol sections significantly deviates within the clock generating unit 328. In such case, the clock generating unit 328 generates a clock with deviated timing. Since the judging unit 327 operates in synchronization with this clock, the judging unit 327 will perform its judgements with a non-optimal judgement timing. This can result in the judging unit 327 making erroneous judgements.